This page is considered an official policy on Wikimedia Commons.(See the list: Policies and guidelines)
It has wide acceptance among editors and is considered a standard that all users should follow. Except for minor edits, please make use of the discussion page to propose changes to this policy.

In all cases the uploader must provide appropriate evidence to demonstrate either that the file is in the public domain or that the copyright owner has released it under a suitable licence. Typically that requires at least that the source of the file be specified,[1] along with the original source where the file is a derivative work. Also, the creator or copyright owner should be identified, if known or reasonably ascertainable. If there is any question, evidence may need to be supplied that the copyright owner has indeed released the file under the given licence.

↑Note that in the case of files found on the Web, this should not be the URL of the file, but the URL of the page containing the file, so that Commons editors can find background information when required.